As is known by those skilled in the art, the camber of a vehicle wheel can determined using a sensor that determines the position of a measurement point at an upper, center location on the tire sidewall. Camber can then be calculated based upon known trigonometric relationships between that measurement point and at least one other point (such as the tire center). As is also known, the caster of a steerable vehicle wheel can be determined based upon the measured camber values at each of two steered positions. This technique is described by D. B. January in "Steering Axis Geometry and Caster Measurement," SAE Publication No. 850219.
In many wheel alignment machines, camber is calculated using the horizontal distance between the upper measurement point and the tire center or other such measurement point. This camber calculation assumes that these two measurement points both lie within a vertical plane that is perpendicular to the rotational plane of the wheel (i.e., perpendicular to the plane in which any arbitrary point on the wheel moves when the wheel is rotating). This vertical plane will hereinafter be referred to as the "vertical measurement plane."
Given this assumption, it is commonly accepted that the actual upper measurement point must be as close as possible to this vertical measurement plane. Otherwise, the position of the measurement point will be influenced by the toe of the wheel and the calculated camber will be incorrect. Thus, the measurement sensors used in wheel alignment machines are typically designed and are often even calibrated so that, for a wheel set at zero toe, the upper measurement point will be as close as possible to the upper, center position on the wheel (i.e., as close as possible to the vertical measurement plane).
One problem with determining caster based upon camber values is that the measured displacements are small and are typically not much larger than the resolution of measurement. Moreover, the influence of the toe angle on the displacements of the measurement points is increased due to the intentional introduction of specific toe angles during the measurement process. Consequently, the caster measurements are less accurate and less repeatable than is considered desirable.
In many non-contact type measurement sensors, this problem is further aggravated by the fact that the shaped light is projected in a predetermined direction that does not change when the wheel is steered left and right. Thus, when camber is being determined for purposes of caster measurement, the point of impingement of the shaped light (i.e., the measurement point) will vary as the wheel is steered between the two left and right positions. This leads to further inaccuracies in the camber (and, thus, caster) measurement.